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    <title>unobs</title>
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    <center>Scilab Function</center>
    <div align="right">Last update : April 1993</div>
    <p>
      <b>unobs</b> -  unobservable subspace</p>
    <h3>
      <font color="blue">Calling Sequence</font>
    </h3>
    <dl>
      <dd>
        <tt>[n,[U]]=unobs(A,C,[tol])   </tt>
      </dd>
    </dl>
    <h3>
      <font color="blue">Parameters</font>
    </h3>
    <ul>
      <li>
        <tt>
          <b>A, C</b>
        </tt>: real matrices</li>
      <li>
        <tt>
          <b>tol</b>
        </tt>: tolerance used when evaluating ranks (QR factorizations).</li>
      <li>
        <tt>
          <b>n</b>
        </tt>: dimension of unobservable subspace.</li>
      <li>
        <tt>
          <b>U</b>
        </tt>: orthogonal change of basis which puts <tt>
          <b>(A,B)</b>
        </tt> in canonical form.</li>
    </ul>
    <h3>
      <font color="blue">Description</font>
    </h3>
    <p>
      <tt>
        <b>[n,[U]]=unobs(A,C,[tol])</b>
      </tt> gives the unobservable form of an <tt>
        <b>(A,C)</b>
      </tt> 
    pair.
    The <tt>
        <b>n</b>
      </tt> first columns of <tt>
        <b>U</b>
      </tt> make a basis for the unobservable
    subspace.</p>
    <p>
    The <tt>
        <b>(2,1)</b>
      </tt> block (made of last <tt>
        <b>nx-n</b>
      </tt> rows and <tt>
        <b>n</b>
      </tt> first 
    columns) of <tt>
        <b>U'*A*U</b>
      </tt> is zero and and the <tt>
        <b>n</b>
      </tt> first columns 
    of <tt>
        <b>C*U</b>
      </tt> are zero.</p>
    <h3>
      <font color="blue">Examples</font>
    </h3>
    <pre>

A=diag([1,2,3]);C=[1,0,0];
unobs(A,C)
 
  </pre>
    <h3>
      <font color="blue">See Also</font>
    </h3>
    <p>
      <a href="contr.htm">
        <tt>
          <b>contr</b>
        </tt>
      </a>,&nbsp;&nbsp;<a href="contrss.htm">
        <tt>
          <b>contrss</b>
        </tt>
      </a>,&nbsp;&nbsp;<a href="canon.htm">
        <tt>
          <b>canon</b>
        </tt>
      </a>,&nbsp;&nbsp;<a href="cont_mat.htm">
        <tt>
          <b>cont_mat</b>
        </tt>
      </a>,&nbsp;&nbsp;<a href="../linear/spantwo.htm">
        <tt>
          <b>spantwo</b>
        </tt>
      </a>,&nbsp;&nbsp;<a href="dt_ility.htm">
        <tt>
          <b>dt_ility</b>
        </tt>
      </a>,&nbsp;&nbsp;</p>
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